Eddie, you are drilling to 7mm, you are using a 7mm barrel trimmer, and therefore presumably you are using 7mm bushes. In which case, you can discount the bushes as being the cause. 7mm bushes, as of course you already know, don't enter the tube, and therefore cannot be held responsible for the tube being mis- shapen, as shown in your photo.
When you say you trim using a pillar drill, are you just using the drill to hold and rotate the trimmer, then offering the blank to the trimmer by hand, or are you holding the blank in some sort if vice, then lowering the chuck for the trimmer to enter the blank tube?
